The present invention relates to fiber reinforced plastic structures suitable for use on load bearing components and having improved energy absorbing characteristics. Specifically, the present invention relates to fiber-reinforced plastic composite tubes suitable for use as structural components and load bearing members. More particularly, it relates to such composite tubes having an improved collapse initiator or collapse trigger means to cause stable collapse of the tube and increased impact energy absorption capability in the event the tube undergoes destructive axial impact.
The high strength, stiffness and low density of fiber-reinforced synthetic material composites recommend such materials for many structural applications in view of their potential for structural weight reduction. Fiber-reinforced plastic materials are characterized by low strain to failure properties ("brittleness"), but it has now been well established that structures made from such materials can have an energy absorption potential as good as or better than those made from metals, even under high loading rates. The major difference between such composites and metal structures in this respect is that metal structures almost invariably collapse by buckling and folding, which involves plastic deformation, whereas fiber-reinforced plastic composite structures almost invariably collapse by fracture processes involving fiber and matrix fracture, fiber/matrix debonding and delamination.
One particularly attractive potential application for fiber-reinforced plastic components is as structural, load bearing motor vehicle body components, such as support beams and the like. In particular, the application of fiber-reinforced plastic composites to the front end structure of a motor vehicle has significant potential, apart from the advantages of weight reduction, due to the possibility of parts integration. Analytical techniques for the design of such composite material load bearing structures are established, as are suitable manufacturing techniques. In many such applications, however, including particularly motor vehicle structural components, the load bearing capacity during structural collapse and the energy-of-impact absorption capacity of the structural component are significant to the question of component design. Studies have been conducted to examine the energy absorbing capability of fiber-reinforced synthetic materials during destructive impact. Thornton, P. H., J. Comp. Mat., Vol. 13, (1979), pp. 247-262; P. H. Thornton and P. J. Edwards, J. Comp. Mat., Vol. 16, (1982), pp. 521-545. Such studies on the axial crushing of fiber-reinforced plastic tubes have indicated that significant energy absorption can be obtained from these materials, under some circumstances exceeding that which can be obtained from metal tubes of similar size. Thus, fiber-reinforced plastic composite structures can provide energy absorption with less weight than comparable metal structures, which is particularly useful in the design and manufacture of motor vehicles in which reduced weight provides improved fuel efficiency.
Energy absorption upon axial crushing of composite tubes is known to be dependent upon the geometry and fiber orientation of the tube and upon the mechanism by which the tube collapses. Tubes which collapse by buckling rather than by crushing, that is, tubes which exhibit "unstable" collapse modes, absorb less energy than those which exhibit "stable" collapse modes. Generally, unstable collapse modes are more likely in rectilinear composite tubes, that is in tubes of rectangular (including square) cross section, than in cylindrical tubes, perhaps because of the presence of strong, rigid corners with weak planar side walls. For tubes having planar walls, the collapse mode is known to be dependent upon tube relative density, that is, the ratio of wall thickness to wall width, fiber orientation, fiber type and temperature. Nevertheless, composite tubes having one or more flat walls are desirable for use as structural load bearing components, espcially for use as motor vehicle components since packaging constraints may not permit an entirely cylindrical cross-section and since other components are more easily secured to flat wall sections.
Attempts have been made to control the collapse mode of composite material rectilinear tubes to improve the energy absorption characteristics thereof. For example, to provide uniform collapse in an axially crushed fiber-reinforced plastic rectilinear tube, attempts have been made to initiate the collapse at one end of the tube. It has been found that to obtain stable crush and high energy absorption, a collapse initiator or collapse trigger means is required to initiate the collapse process. If the trigger means is absent or does not function, then the structural collapse can occur in an unstable mode (e.g., the tube can break into a few large sections) with low energy absorption. Specifically, it is known to provide a collapse trigger by providing one or another configuration to the end of the tube. More specifically, it previously was known to provide a collapse trigger for glass or graphite fiber reinforced plastic rectilinear tubes in the form of a chamfered or bevelled tube end surface, as shown in FIG. 1. The reduced tube end surface which makes contact upon initial impact provides stress concentration necessary to form numerous starting cracks at lower applied loads. Cracks nucleated at the bevel then propagate (on further loading) into the body of the tube causing interlaminar fracture, fiber fracture and fiber/matrix separation. In addition to causing the collapse to propagate in a stable manner, the initiator or trigger means can reduce the initial load at which the collapse commences. In a fiber-reinforced plastic rectilinear tube used as a structural component of a motor vehicle, mounted horizontally in a fore-and-aft direction, for example, the effect of reducing the initial load is to produce a less aggressive impact, that is, to moderate the shock of a tube-collapsing impact.
It is an object of the present invention to provide a fiber-reinforced plastic tube having collapse characteristics significantly improved over those of known fiber-reinforced plastic rectilinear tubes. In particular, it is an object of the invention to provide a structural composite tube having at least one flat wall and adapted to be load-bearing, which tube has improved energy absorption capability upon being axially crushed.